Polyesteroverepoxy

By Jeff Wright Applying Most production fiberglass boats are made berglass are chemical or primary bonds as op- with polyester resin. WEST SYSTEM epoxy is posed to mechanical or secondary bonds. The polyester a wonderful material for repairing polyester fi- manufacturing process for polyester boats has berglass boats. One reason for this is the abil- been developed so that a chemical bond exists gelcoat to ity of epoxy to form a stronger mechanical between the gelcoat and the laminate. When a bond to a damaged laminate than polyester polyester boat is built, polyester gelcoat is first WEST SYS- resin. Epoxy also provides a better moisture sprayed onto the mold surface. The first layer TEM® epoxy barrier than polyester resin. of the laminate is then applied to gelcoat, Our customers’ fiberglass repairs range from which has not completely cured. The two lay- repairs made filling a minor screw hole to completely re- ers eventually cure together with a chemical building the bottom of a boat that ran bond between them. BELOW the aground. In most cases, a final finish needs to Applying gelcoat to a cured laminate relies on be applied over the repair. If the repair is a mechanical bond. Because of the difference water line above the waterline, high-gloss paint or in curing chemistry, it is not possible to gelcoat are often used to reproduce the origi- achieve a chemical bond between epoxy and nal appearance. If the repair is below the wa- polyester gelcoat. We developed some tests, to terline, antifouling paint is most often applied determine whether or not the mechanical over the repaired area. However, customers bonds achieved between gelcoat and properly often want to restore the original gelcoat fin- prepared, cured epoxy were strong enough to ish to below-waterline repairs where the boat achieve a durable repair. is kept in the water and no antifouling paint is Testing applied. These customers are concerned about the gelcoat’s long term ability to adhere to the The test program we developed was designed area repaired with epoxy. There are good rea- to determine the effect of moisture on the ad- sons for this concern, but if you understand hesion of polyester gelcoat applied over fully the materials involved and follow recom- cured epoxy. The tests were done on a gelcoat mended repair procedures, you can success- finish over a simulated repair made with fully apply a gelcoat finish to a boat bottom WEST SYSTEM epoxy. The test included a va- repaired with WEST SYSTEM epoxy. riety of finishing processes. In addition, we varied the length of time the epoxy was al- In repairs above the waterline, gelcoat applied lowedtocurebeforebeingsubjectedtohigh over properly prepared WEST SYSTEM ep- moisture conditions. For comparison, we also oxy has a great track record. Many boats have tested gelcoat adhesion to a polyester laminate been repaired this way and are performing using West Marine Boaters Resin™. well in a variety of climates. Boats that are dry sailed or trailered have also had good success Two popular gelcoats were used, Minicraft™ with gelcoats on underwater epoxy repairs. and Evercoat™. Interestingly, Evercoat states Concern about adhesion occurs in situations on their can that it is not to be used over ep- where boats are in the water constantly and oxy. Although we often suggest following the need to have a high level of finish on their manufacturer’s instructions, we believe, in this bottoms that requires gelcoat. Such a situation particular case, the manufacturer is overly might occur with a boat kept in a pristine lake conservative. As with many products, the sur- where antifouling paint is often not used. face must be prepared properly; if it is not, Since the polyester gelcoat is not very good at then poor curing and adhesion may result. preventing moisture from permeating through Polyester materials can be affected by amines to the laminate, there has been a concern that, in the epoxy hardener. If the hardener has not over time, the gelcoat will detach from the fully reacted with the epoxy resin or the amine more moisture resistant epoxy layer. blush is not removed from the cured surface, problems can occur. Proper surface prepara- Understanding the materials tion will prevent these problems. (See 002-550 Polyester resin laminates achieve their highest Fiberglass Boat Repair & Maintenance, Section strength when the bonds between layers of fi- 2.2.1 for details.)

1 EPOXYWORKS Number 22, Winter 2004 Preparing the test panels u =36hourepoxycure n =7dayepoxycure The test panels were laminated from three lay- 1500 ers of 738 Biaxial Fabric (15 oz biaxial with .8 Minicraft™ Gelcoat over 105/206 742 6oz Glass oz mat) wet out with 105 Resin and 205 1000 Hardener. Panels were covered with 879 Re- Cloth with 105/206 lease Fabric and allowed to cure. This laminate sealer coat provided a stiff panel that simulated a patch 500 used to repair the bottom of a fiberglass boat. 1500 After the Release Fabric was removed, a vari- Minicraft Gelcoat over ety of finishing processes were applied to the 105/206/407 Low-Den- 1000 panels. These included using 742 Glass Fabric sity Filler with 105/206 (6 oz), 407 Low-Density Filler, and 410 sealer coat Microlight™. All of the test panels had a sealer 500 coat of 105 Resin/206 Slow Hardener to fill the weave and seal the scratches in the sanded 1500 fairing compounds. Slow Hardener was cho- Evercoat™ Gelcoat over 105/206 742 6oz Glass sen because its slower reaction time would be Cloth with 105/206 1000 more likely than 205 Fast Hardener to reveal sealer coat any adhesion problems. 500 The gelcoat was applied using a pressure pot type spray gun. It was thinned using 10% Ace- 1500 tone by weight catalyzed at 1% with MEKP Evercoat Gelcoat over (Methyl Ethyl Ketone Peroxide). After the 105/206/410 Microlight™ 1000 gelcoat cured for two weeks, PATTI (Pneu- Filler with 105/206 sealer matic Adhesive Tension Testing Instrument) coat studswereappliedtothesurface. 500 One set of PATTI studs were pulled for a base- 1500 line value. Panels were then placed in our 100% humidity/100°F environment room. Minicraft Gelcoat over The conditions in this room accelerate any ef- Polyester resin 742 6oz 1000 fect that moisture will have on the laminate. Glass Cloth The remaining PATTI studs were pulled at 500 three, six and nine weeks to determine the long term effect of moisture on the bond. 1500 Figure 1 (right) lists the different material Evercoat Gelcoat over combinations that were sprayed with gelcoat. Polyester resin 742 6oz 1000 The graph for each combination shows the Glass Cloth tensile load required to pull the stud off the panel at four time intervals, illustrating the 500 trend of the bond strength when the gelcoat is Tensile force applied after allowing the laminate to cure for required to pull 36 hours and 7 days. Each data point repre- stud (PSI) Initial 3 weeks 6 weeks 9 weeks Period of exposure after gelcoat application sents the average of five stud pulls. Conclusion Figure 1 Gelcoat Adhesion Test The overall effect of moisture on the gelcoat’s bond to both the polyester and epoxy panels Graphs above show the nine-week exposure may not simulate what was negligible. The graphs show very little trend of gelcoat bond could happen after many years, but it does in- change in the adhesive strength, and much of strength at four time in- dicate that the gelcoat bond to epoxy lami- tervals. Each data point the variance is within the tolerance of the test nates is a good bond and should perform well. is the average tensile method. When the studs were pulled, the fail- load required to pull ures occurred in the fairing compound, cohe- Our test clearly demonstrates that polyester studs from the surface sive failure of the gelcoat, or in the bond gelcoat will bond to a properly prepared of different material between the gelcoat and laminate. Since these WEST SYSTEM® epoxy as well as to a cured combinations that were three modes of failure occurred under similar polyester laminate. This test confirms that sprayed with gelcoat. loads, it indicates that the strength of the bond polyester gelcoats can be applied over cured is close to the cohesive strength of both the WEST SYSTEM epoxy on repairs below the gelcoat and the fairing compound. The waterline. n

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